Seeking Cyanobacterial Cellulose

By Jerry W. Kram

Dozens of companies and research labs are racing to produce biofuels from algae. Most are focused on extracting oil from green algae for biodiesel, but two University of Texas researchers are taking a different track. They have fused new genes into cyanobacteria species to make them prodigious producers of cellulose and sugars, an attractive prospect for the ethanol industry.

Cyanobacteria are photosynthetic bacteria often referred to as blue-green algae. They grow even more rapidly than green algae, doubling in just four hours compared with 24 hours. Some of these organisms produce cellulose, but R. Malcolm Brown and David Nobles Jr. took a species that doesn't normally produce cellulose and added genes from an Acetobacter species that allowed them to synthesize cellulose. "We have been studying cellulose biosynthesis for 40 years, and the idea was to get the cellulose biosynthesis into the cyanobacteria that normally do not make cellulose," Brown says. The researchers also created modified organism strains that secrete sucrose and other simple sugars.

When they analyzed the results, Brown and Nobles found that the cellulose produced by the cyanobacteria was easier to convert into biofuels than the cellulose produced in plants. The organisms produce a gel-type of cellulose instead of the crystalline type found in the cell walls of plants. "There are a lot of other things in trees, such as lignins and hemicelluloses that need to be removed," Brown says. "The more crystalline they are, the harder they are to break down. So when we found our cellulose had zero crystallinity and very low molecular weight, that made it easier to break down."

The cyanobacteria can grow in either fresh or briny water, Nobles says. They only require a few micronutrients and fixed nitrogen to thrive. Unlike green algae, the cyanobacteria produced the desired products with little environmental manipulation. "We have to tweak a few things, but nothing that inhibits their growth and nothing costly," Nobles says. In the lab, the cultures were able to produce large amounts of cellulose from carbon dioxide in the atmosphere, but it may be possible to increase their productivity by enriching them with additional carbon dioxide, he added.

Nobles says another advantage of using the cyanobacteria is that they secrete the cellulose and sugar into their surroundings where it can be harvested without sacrificing any of the organisms. Green algae has to be collected and their cells broken open to get at the oil inside. "You don't have to harvest the cells, and you don't have to put any energy into the extraction," Brown says. "You eliminate the two most costly steps in the production."

The next step for the researchers is to scale up their work in the lab to a larger facility. "We are working right now to get this out of the laboratory and into a demonstration-scale facility," Brown says. "No one is going to invest a huge amount in this, and the federal government isn't going to be that interested until they know it really can work at a demonstration level." There are many options, but the researchers say they may consider using photobioreactors to forestall objections to growing a genetically modified organism in open ponds. "It is likely that we will also require a closed system because we will be growing an organism that is secreting sugars," Nobles says. "That would create problems in an open pond."